Nanostructured materials have shown promise as electrodes because of their high storage capacity and high rate performance in terms of gravimetric energy density. Unfortunately, these positive effects are countered by a generally low packing density, which results in poor volumetric storage capacity. On the other hand, mesoporous electrode materials have been shown to exhibit high packing density but that positive aspect is offset by their inherent low electrical conductivity. That is, mesoporous materials for electrochemical use are usually synthesized as micrometer-size particles composed of nano-scale grains and having nanopores throughout in the range of 2-10 nm. This results in ready access for lithium ions to the electrode surface from electrolytes and facile Li+ transport within the grains. However the electrons within the mesoporous particles need to be transported across a few microns to the nearest conductive carbon additive or current collector. Thus, in mesoporous materials a short transport length for Li-ions accompanied by a long transport length for electrons is observed, which is a limiting factor for high storage performance. To ameliorate this problem, electronically conductive carbon or RuO2 (a metallic oxide that allows both Li+ or e− to migrate) can be coated on the interior or exterior surfaces of the mesoporous material, which can result in excellent storage performance at high rates. For example, Guo, et al., Chem. Commun., 2005, 2454, reported a mesoporous TiO2 with a surface area of about 130 m2g−1 coated with RuO2 that exhibits storage capacity of 190 and 125 mAh/g at 1 and 10 C rate. Though the preceding technique is widely employed to achieve high storage performance, there remains a need for electrode materials with superior storage capacity at high C-rates without complex surface coatings of conductive materials so as to result in less complicated, more economical lithium-ion batteries. The current invention provides such an electrode material.
In lithium-ion batteries, chemical energy can be reversibly stored through recharging by homogeneous intercalation and deintercalation reaction without significant structural changes. For example, lithium ion batteries used in laptop, mobile phones etc., comprised of LiCoO2 as the cathode and graphite as the anode make use of the rocking chair concept of lithium insertion. During the charge operation, Li+ ions from LiCoO2 transfers to the graphite anode through the non-aqueous electrolyte to form LiC6 while electrons flow in the reverse direction through the external circuit. During the discharge operation (when delivering power to an appliance) the reverse occurs. Thus the Li+ shuttle between the two electrodes during charge-discharge cycling is facilitated by the layer-type crystal structure of the electrodes. The high reversibility of the electrochemical process is caused by the soft insertion/extraction of Li+ in these host lattices.
Lithium ion battery provide many advantages such as high open circuit voltage (˜4 V), excellent cyclic performance (more than 3000 charge-discharge cycles) and high coulombic efficiency (95%), but they generally exhibit limited lithium storage capacity. Only 0.5 Li could be removed from LiCoO2 resulting in Li0.5CoO2 (137 mAh/g using a half cell of LiCoO2 versus Li metal) and 1 Li+ could be stored in graphite in the form of LiC6 (370 mAh/g using a half cell of graphite versus Li metal). Since most of today's high performance portable microelectronic devices demand high energy density, there is a great interest in increasing the storage capacity of Li in both cathode and anode materials.